Conventionally, many zoom lens systems have been proposed that comprise two, i.e., a negative and a positive, lens groups. For example, Patent Document 1 identified below proposes a zoom lens system wherein a second lens group is composed of a front group and a rear group and the front group is used as a focusing group to achieve compactness. Inconveniently, however, the zoom lens system proposed in Patent Document 1 produces large curvature of field at the periphery of the image field, and produces large chromatic coma.